Transformers are magnetic components that have many uses, such as for transforming voltages and for providing isolation between the circuits on the primary and secondary sides of the transformer.
Recently, planar magnetic components have become widely used in power electronic devices, such as switched mode power supplies (SMPSs). An example of an SMPS constructed with planar magnetic components is shown in FIG. 1.
A planar magnetic component comprises two pieces of magnetic material (usually referred to as “cores”, but sometimes referred to as “half-cores”) which are used with one or more flat coils (also referred to as turns) printed on a printed circuit board (PCB). Typically, one core is positioned above the one or more coils and a second, identical, core is positioned below the one or more coils, with the cores being connected together through at least one hole in the PCB.
Referring to FIG. 2, by way of example, the parts of a planar magnetic transformer are shown unassembled. An upper core 11 and a lower core 12 are provided respectively above and below a multi-layered PCB 13. Cores 11 and 12 are identical E-plane cores. The layers of the PCB comprise at least one hole to allow the central part of each core to extend into the PCB 13. Typically the PCB 13 would also contain holes, not shown in FIG. 2, to allow the outer wings of the “E” of each core to also extend into the PCB 13. Printed tracks on the layers of the PCB 13 provide coils round the centre part of the core as well as input and output connections to the transformer. The coils on each layer either provide a winding for the primary side of the transformer or a winding for the secondary side.
The upper core 11 and the lower core 12 are attached to each other by the mechanical clip 14. In the arrangement shown in FIG. 2, the mechanical clip 14 extends around the edges of the PCB 13 and the ends of the mechanical clip 14 are attached to the recesses 15, 16 in the top surface of the upper core 11. Although a single mechanical clip is shown, two mechanical clips may alternatively have been used with separate clips attaching to respective ends of the upper and lower cores. The two cores may alternatively have been glued together instead of mechanical clips being used.
For a planar magnetic transformer, primary and secondary windings are provided by using a multi-layered PCB such as the arrangement shown in FIG. 2. In the arrangement shown in FIG. 2, a plurality of coils, or turns, are provided on each layer of the PCB. Alternatively, only a single coil, or turn, may be used on each layer.
The transformer shown in FIG. 2 has the layers comprising printed tracks, that provide the coils of the transformer, fully interleaved. That is to say, for layers within the structure (i.e. those between the top and bottom layers), each layer that provides a coil of the primary windings of the transformer is directly adjacent, i.e. above and below, two layers that provide coils of the secondary side of a transformer. Similarly, each layer that provides a coil of the secondary side of the transformer is directly adjacent to layers that provide coils of the primary side of the transformer. In this way, no layer providing a primary winding is adjacent another layer providing a primary winding. Similarly, no layer providing a secondary winding is adjacent another layer providing a secondary winding.
It is known to fully interleave the windings of the primary and secondary sides of a planar transformer. Fully interleaving the windings of the primary and secondary sides of the planar transformer improves the magnetic coupling between the primary and secondary sides and reduces flux leakage compared to an arrangement in which there is no interleaving between the primary and secondary windings.
FIG. 3 is a vertical cross-section of a multi-layered PCB showing the windings of a fully interleaved transformer with twelve layers. The outer layers (i.e. the top and bottom layers in FIG. 3) each have a metal thickness to. The inner layers each have a metal thickness ti, with ti greater than to. The metal used to form the layers is typically copper.
Between each pair of the metal layers electrical isolation is provided. The isolation material is typically a plastic substrate. The thickness of the isolation material between the layers is hh. FIG. 3 shows a known arrangement in which the spacing hh between each of the layers is the same throughout the vertical cross-section of the PCB.
A problem experienced by the fully interleaved PCB shown in FIG. 3 is that the parasitic capacitive coupling between the primary and secondary windings is large. A way of reducing the parasitic capacitive coupling is to increase the thickness of the isolation material between the layers so that the metal layers within the PCB are spaced further apart from each other. However, increasing the spacing between the layers results in the parasitic leakage inductance increasing.
Another requirement of such a planar magnetic transformer is for it to maintain good isolation between the primary and secondary sides of the transformer. The isolation material and spacing between the primary and secondary windings must therefore provide the required isolation properties of the transformer. A standard isolation voltage is 2250V between the primary and secondary sides. This imposes strict requirements on the isolation material and the distances between the primary and secondary windings.
A known manufacturing process of a multi-layered PCB for a planar magnetic transformer is described below with reference to FIG. 4.
A solid plastic substrate, also referred to as a laminate, is typically used as the isolation material. Tracks of the PCB are formed on the upper and lower surfaces from the substrate either by a subtractive process from a substrate with upper and lower surfaces entirely covered by metal or by an, additive process onto a substrate without metal coverings on its upper and lower surfaces.
Several such substrates are then bonded together by applying a fluid pre-preg and then applying pressure and heat.
The upper and lower layers of the PCB are then added using a pre-preg process again and forming the thinner upper and lower metal layers thereon.
Holes are then drilled in the PCB for vias between the layers and, if not already present, cuts are made to allow the core and wings of a transformer to extend into the PCB. The via holes are then electro-plated to form vias.
FIG. 4 shows a vertical cross-section of an entire PCB at stages during the manufacture of a PCB with six metal layers.
In FIG. 4, process 1 shows the bonding between multiple substrates, with metal tracks on their upper and lower surfaces, with pre-prep. Process 2 shows the subsequent adding of the upper and lower metal surfaces of the PCB.
Throughout the present document, the thickness of a layer is the dimension of a layer in a direction that is normal to the upper or lower surface of one of the planar layers.
As is clear from FIG. 4, the layers of pre-preg are thicker than those of the substrate.
Layers formed by a pre-preg process cannot be formed as thin as layers of substrate due to the nature of the pre-preg process.
Standard manufacturing processes have a ±10% tolerance on the thickness of the layers.
With standard manufacturing processes, the minimum substrate thickness that can be designed for is about 100 μm and the minimum pre-preg thickness that can be designed for is about 150 μm. Thus, the minimum actual substrate and pre-preg thicknesses may be as low as 90 μm and 135 μm, respectively, due to the ±10% manufacturing tolerance.
The average thickness of the pre-preg layer is required to be thicker than that of the substrate layer in order for it to be possible for the pre-preg to fill in the gaps between the printed tracks in the metal layers.
In order to provide an isolation voltage of 2250V between the primary and secondary sides of the transformer, the pre-preg isolation material should be designed to have a minimum thickness of 175 μm. That is to say, due to the manufacturing tolerance, the pre-preg isolation material meets the 2250V requirement if it has a thickness of at least 157.5 μm.
Accordingly, the isolation material in the fully interleaved transformer shown in FIG. 3 must be designed to be at least 175 μm thick, hh≧175 μm, and the minimum manufacturable thicknesses of substrate and pre-preg cannot be used.
With regard to the thickness of the metal layers, this is specified in terms of ounces of copper, where:
                              1          ⁢                                          ⁢          oz                =                ⁢                  the          ⁢                                          ⁢          thickness          ⁢                                          ⁢          of          ⁢                                          ⁢          1          ⁢                                          ⁢          ounce          ⁢                                          ⁢          of          ⁢                                          ⁢          copper          ⁢                                          ⁢          when          ⁢                                          ⁢          rolled          ⁢                                          ⁢          out          ⁢                                          ⁢          over                                                ⁢                  an          ⁢                                          ⁢          area          ⁢                                          ⁢          of          ⁢                                          ⁢          1          ⁢                                          ⁢                      ft            2                                                  =                ⁢                  35          ⁢                                          ⁢          μm                    
In FIG. 3, to=2 oz and ti=4 oz.
Throughout the present document the height of a PCB is the dimension of the PCB in a direction normal to the upper or lower surface of one of the planar layers.
The total height of the PCB shown in FIG. 3 is:
                              H          ⁢                                          ⁢          1                =                ⁢                              (                          10              ×              4              ⁢                                                          ⁢              oz                        )                    +                      (                          2              ×              2              ⁢                                                          ⁢              oz                        )                    +                      (                          11              ×              175              ⁢                                                          ⁢              μm                        )                                                  =                ⁢                  3.465          ⁢                                          ⁢          mm                    
A problem with the above-described known arrangement of a fully interleaved stacked-up multi-layered PCB, is that the height of the PCB is relatively large and this results in poor thermal conductivity from the transformer.
In addition, increasing the metal thickness, or number of layers, will increase the total height of the PCB further and thereby reduce the thermal conductivity even more. Poor thermal conductivity results in the planar magnetic transformer being unsuitable for high power applications.